Electrochemical sensors, such as The Foxboro Company's 871PH Sensor, include a glass or metal sensing electrode and a reference electrode which are immersed into a process solution to measure the solution's pH (acidity) or ORP (oxidation reduction potential). Typically, a post extends from the sensor into the process solution and serves as a common electrical ground. The sensing electrode produces an electrical potential proportional to the hydrogen ion activity (for pH sensors) or oxidation activity (for ORP sensors). The reference electrode completes the circuit and provides a stable reference for the sensing electrode. These two electrodes constitute an electrolytic cell having a continuous millivolt output that is proportional to the pH or ORP of the solution.
Continuous on-line pH or ORP measurements are very important for many liquid processes. For example, in the manufacture of fine and specialty paper, tight control over the pH value of the pulp is necessary to ensure total paper quality. Typical ORP applications include treatment of chromate and cyanide wastes produced by plating, chemical, and metal treatment plants. ORP monitoring enables industry to determine reaction end points to meet today's stringent environmental requirements. A problem often encountered with these sensors, however, is that particulates in the process solution, for example the pigments, fillers, and dyes used in the manufacture of paper, bond to the sensor surfaces. Over time the electrodes and the common ground become coated and the measurements may become unreliable. To avoid removing the sensor for external cleaning of the electrodes and the common ground and to provide continuous, accurate pH and ORP sensing, two entirely different electrode cleaning systems were developed: an ultrasonic cleaning system and a mechanical cleaning system.
The ultrasonic cleaner consists of a disk-shaped transducer positioned very close to the sensing and reference electrodes. Ultrasonic energy waves are generated to keep particulates in suspension and prevent them from settling on any surface within reach of the ultrasonic waves. Tests have demonstrated, however, that very fine particles do settle on and eventually coat the electrode and common ground surfaces. This necessitates continued periodic removal of the electrodes for external cleaning.
Mechanically cleaning the electrodes is an entirely different approach. This approach typically includes a rotating brush which continuously sweeps clean the surface detection areas of both electrodes. The electrodes used in this cleaning system have flat detection surfaces. Over time, however, these brushes accumulate filler material, pigments and other matter which prevent the brushes from properly cleaning the electrodes.
Electrode sensitivity problems were also encountered with these cleaning systems when large amounts of stock accumulated around and between the electrodes. To prevent this accumulation and to minimize the down time required for external cleaning of the electrodes, the sensor is commonly placed on a sample line connected to the main stream of process solution. Periodically, the sample line is isolated from the main line and flushed to dislodge and remove all of the accumulated stock and filler away from the electrodes and cleaner brush. After flushing, the sample line is again connected to the main stream of process solution. During this time, however, measurements of the process fluid are lost.